It is known to enclose sensitive electronic equipment in an electrically conductive cage, a Faraday cage, to obtain good electromagnetic compatibility (EMC) with respect to said equipment, e.g. protection against electrostatic discharges (ESD).
In those situations when duplicate systems are used, two such pieces of equipment, will normally stand side-by-side so that the first piece of equipment is able to take over the functions of the second piece of equipment should this second piece of equipment malfunction or require inspection. It is, of course, possible to enclose both pieces of equipment in one and the same cage, thereby protecting both pieces of equipment. However, problems arise when the first piece of equipment needs to be repaired or serviced. In order to obtain access to this equipment, it is necessary to open the cage, which will impair the EMC properties with respect to the second piece of equipment, therewith risking disablement of the second piece of equipment and therewith a total breakdown. It is therefore necessary to enclose each piece of equipment in its respective cage.
In order for equipment that has good EMC properties in general to communicate with other equipment via cables for instance, it is necessary to provide holes in the cage that encloses the equipment and that constitutes the actual basis for said good EMC properties. Such holes enable radiation to leak into and out of the cage, resulting in poorer EMC properties.
When two cages stand side-by-side, electrical discharges, ESD, from one piece of equipment in the first cage are able to pass out through the hole therein and wander through the air or along electric conductors and in through the hole in the second cage and there disable the equipment enclosed by this second cage. This is, of course, highly unfortunate and presents a specific problem to the application of duplicated systems.
There are many different causes of electrostatic discharges, ESD. For instance, the equipment may be subjected to such discharge by a person coming into contact with the equipment or solely being present in the proximity thereof. In order to prevent such an occurrence, there are often provided special grounding or earthing points at which a person is able to connect up an electrically conductive arm band and therewith obtain the same ground, or earth, as the equipment, thereby preventing an ESD. Unfortunately, an ESD will sometimes occur before the person concerned has been able to ground his body at a grounding point, which may have catastrophic consequences in the case of sensitive equipment. A second person can cause an ESD, even though the first person is grounded or earthed. If the first person has had time to open the cage, the EMC properties of the equipment enclosed therein will be greatly impaired, and even if the actual discharge does not cause problems, the electromagnetic radiation caused by the discharge may play havoc and create problems with respect to nearby equipment, by passing through holes that carry electric cables.
Computers that are used for switching purposes in telephony are often duplicated, so that if the first computer is disabled, the other computer will take over. When repairing the first computer, it is extremely important that no electrostatic discharges caused by a careless technician will also disable the other computer, which would result in a total breakdown in operations. The problem is particularly serious in respect of new types of IC circuits that are extremely sensitive to static discharges.